1. Field of the Invention
The present invention relates to a photothermographic material and a method for manufacturing the photothermographic material. More specifically, the invention relates to a photothermographic material which exhibits high image quality and excellent image storability, and a method for manufacturing the same.
2. Description of the Related Art
In recent years, in the field of films for medical diagnosis, there has been a strong desire for decreasing the amount of processing liquid waste from the viewpoints of protecting the environment and economy of space. Technology is therefore required for light sensitive photothermographic materials which can be exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications and for use in photographic technical applications. The light sensitive photothermographic materials do not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly thermal processing system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are known. Photothermographic materials generally comprise an image forming layer in which a catalytically active amount of photocatalyst (for example, a silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images, are dispersed in a binder. Photothermographic materials form a black silver image by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region. This system has been described in many documents, and the Fuji Medical Dry Imager FM-DPL is an example of a practical medical image forming system using a photothermographic material that has been marketed.
Thermal developing processing does not require the processing solutions used in wet developing processing, and has an advantage in that processing can be carried out easily and rapidly. However, there are still problems to be solved with respect to the thermal developing processing, which do not occur in wet developing processing. One of them is a problem relating to dyes. Photosensitive materials commonly incorporate dyes in order to adjust a color tone, to provide a light filter function, or to prevent halation or irradiation.
It is important to fix a dye in a specified layer, and as described in Japanese Patent Application Laid-Open (JP-A) Nos. 9-146220 and 11-228698, addition of a water-insoluble dye in the form of a dispersion of solid fine particles has commonly been carried out. All patents, patent publications, and non-patent literature cited in this specification are hereby expressly incorporated by reference herein. Further, when the dye is allowed to be decolored, a decoloring agent is also added in the form of a dispersion of solid fine particles. However, ordinarily, since the solid fine particles have a broad absorption spectrum and cause light diffusion due to the particles, there has been a problem in that turbidity of the film is increased or the like.
In silver halide photosensitive materials for wet developing processing, water-soluble dyes from which a dye having a favorable absorption spectrum and a saturated color can easily be selected have ordinarily been used. In wet processing processes, decoloration by various types of processing solutions or removal of the dye from the photographic photosensitive material by being eluted into the processing solution can easily be performed. However, in the photothermographic material, since the dye remains in a film as is, colorization can only be performed in a limited range. Further, since the water-soluble dye is not fixed to a specified layer and is diffused in many adjacent layers, an effect of preventing halation or preventing irradiation can not efficiently be exerted, and further, as a result of an increase of an amount to be added, residual color is deteriorated. Particularly, a dye for use in color tone adjustment is used in an amount sufficient for obtaining a favorable image tone thereof. Therefore, when the coloration by such dye as described above becomes uneven, a resultant color unevenness is sensitively felt, and as a result, evenness in coloration becomes an important problem.
The image to be obtained by a photothermographic material is handled and stored in various types of environments. In order to allow coloration by the dye under these environments to be always uniform and to maintain stable image tone, a conventional coloring method cannot be said to be sufficient, and further improvement has been required.